I've seen tons of examples of math modeling where parallel lines arise and have a natural interpretation but I could use help in finding ones where the product of the slopes equals -1 is useful in modeling other than in doing bare geometry problems algebraically. I'd appreciate an example where this fact was used in such a way that covering this material didn't seem tangential (please forgive the pun) to a modeling perspective for algebra. I could try to embellish a problem that tries to find the tangent line to a circle or circular cylinder with a specific given radius but as of now I'm trying to do better. Ideally this should be no higher than the intermediate algebra level.